The present invention relates to a demodulator for a multi-level quadrature amplitude-modulated carrier wave and, more particularly, to a demodulator for a multi-level quadrature amplitude-modulated carrier wave which is furnished with an automatic gain control (AGC) circuit.
In the art of microwave digital signal transmission systems, a 16-level quadrature amplitude modulation system (abbreviated as 16 QAM hereinafter) or like multi-level quadrature amplitude modulation system is attracting increasing attention as an efficient modulation system which makes effective use of the frequency band, and development of such a system is now under way. This modulation system is a kind of amplitude modulation system. A demodulator applicable to such a modulation system comprises, for example, phase detectors, a carrier synchronizing circuit and analog-to-digital (A/D) converters. Demodulated signals provided by quadrature phase detection at the phase detectors are individually compared in amplitude with a plurality of predetermined reference levels and thereby discriminated to reproduce digital signals. Therefore, it is a primary requisite for accurate discrimination of demodulated signals that the output levels of the phase-detected demodulated signals be always controlled to correct level relative to the reference levels, regardless of changes in a received signal level, circuit gain, etc. To implement this requisite, the demodulator is equipped with an AGC circuit.
AGC circuits of the type described include one which amplifies a quadrature amplitude-modulated carrier wave in response to a control signal derived from outputs of multilevel discriminators comprising A/D converters, which are connected to output terminals of pnase detectors via base-band (BB) amplifiers, as disclosed in Japanese Patent Unexamined Publication No. 57-131152/1982. Typical of such an AGC circuit may be a circuitry which controls the gain of an intermediate frequency (IF) amplifier. A prior art circuit of the type controlling the IF gain has a sufficiently wide control range over fluctuations in received signal level and other factors whicn commonly effect both the quadrature modulated components. However, it is unable to make up for changes in the sensitivity of phase detectors assigned one to each of the quadrature demodulated components or in the gains of the baseband amplifiers which might result from temperature variations, deterioration due to aging, etc. The result is the liability to erroneous discrimination, or discrimination errors.